In a number of currently used X-ray imaging systems, such as a product of the General Electric Company known commercially as the Revolution XQ/i, an arrangement of adjustable collimator blades is provided to enable the field of view, or Active Imaging Area, to be symmetrically adjusted. As used herein, Active Imaging Area (AIA) refers to a rectangular area, at a specified distance from the tube of an X-ray imaging system, which receives incident X-rays projected by the tube. The dimensions of the AIA generally are adjustable to coincide with the dimensions of the detection area of an X-ray imaging medium, such as an X-ray film or a digital solid state X-ray detector, which is positioned to receive X-rays after projection through a patient or the like. Alternatively, the AIA may be adjusted to coincide with only a portion of the detection area of the medium.
In a symmetrically adjustable collimator, two pairs of spaced-apart collimator blades are mounted on the X-ray tube, one pair to adjust the vertical dimension (or length) of the AIA, and the other pair to adjust the horizontal dimension (or width) thereof. Thus, by rotating a small wheel or dial mounted on the collimator, the two collimator blades associated with the vertical dimension may be smoothly adjusted, to selectively increase or decrease such dimension. Assuming that the tube and detector are not manually positioned with respect to each other, this collimator adjustment is symmetric in that for any setting of the collimator blades, the distance or spacing between the focal spot center line and the top edge of the AIA will be equal to the spacing between the focal spot center line and the bottom edge of the AIA. The focal spot center line coincides with the axis of the projected X-ray beam.
Horizontal adjustment of the AIA is likewise symmetric, in that for any setting of the pair of collimator blades associated therewith, the spacings between the focal spot center line and the right and left edges of the AIA, respectively, will also be equal.
Symmetrically adjustable collimation, as described above, has been found to be very beneficial in an X-ray system. Such arrangement enables an X-ray technician or other user to position a patient in front of the X-ray detector, and to then quickly adjust dimensions of the X-ray beam as required for a particular patient and detector. Symmetric collimation is considered to be particularly useful in connection with digital solid-state X-ray detectors and with systems which utilize an auto tracking tube to enhance user productivity. In a system having the auto tracking tube feature, movement of the detector will result in an automatic corresponding movement of the X-ray tube, in order to maintain a prespecified relationship between the tube and detector. In an X-ray system such as the Revolution XQ/i, referred to above, the tube stand is provided with motorized travel, and automatically tracks the image center on the detector. This feature greatly enhances productivity in setting up an X-ray system for imaging, since a user only needs to manually position the detector with respect to a patient, and the tube will be positioned automatically. However, there is increasing interest in chest imaging, wherein the chin of a patient is positioned near the top of a detector, and the X-ray exposure is limited to the chest region and to the upper abdomen. At present, such imaging requires use of a lead shield or lead apron for the abdominal area, and manual positioning of the X-ray tube and the collimator. While an X-ray system with symmetric collimation may be adaptable for chest imaging, such adaptation may adversely affect image quality and user productivity, and may not take full advantage of auto tracking tube. In chest imaging it is desired to reduce the bottom or lower part of the AIA, without reducing the upper part thereof.
The invention is generally directed to collimator apparatus for an X-ray imaging system comprising an X-ray tube disposed to project an X-ray beam, and a detector which is spaced apart from the X-ray tube for receiving X-rays of the beam within an Active Imaging Area. The apparatus of the invention comprises a first collimator positioned along the path of the beam, the first collimator being vertically adjustable and operable to symmetrically collimate the beam. The apparatus further comprises a second collimating device positioned between the first collimator and the detector. The second collimating device is operable to limit the uppermost rays of the symmetrically collimated beam which reach the detector, and which thus define the upper edge of the AIA on the detector, to a predetermined upper boundary. At the same time, the second collimating device allows the lowermost rays of the symmetrically collimated beam reaching the detector, defining the lower edge of the AIA, to have a lower boundary which is determined by selective adjustment of the first collimator. Thus, the upper edge of the AIA on the detector is fixed, while the lower edge can be raised or lowered by means of the adjustable first collimator.
In a preferred embodiment of the invention, the second collimating device comprises a collimator blade fixably positioned to lie in the path of an upper portion of the symmetrically collimated beam, to prevent X-rays of the upper portion from reaching the detector. The first collimator comprises upper and lower vertically adjustable collimator blades, respectively positioned so that the beam passes therebetween The adjustable collimator blades are moved toward or away from each other, to vary the width of the collimated beam in a vertical plane, while the collimated beam remains vertically symmetrical. Preferably, the adjustable collimator blades are positioned within a housing, and the fixed collimator blade is fixably joined to the housing.